This invention relates to an actuator, driven by a piezo-electric element that expands and contradicts when subjected to a voltage, having a temperature compensation element with a shape memory effect alloy to compensate for the effects of temperature on the expansion of the piezo-electric element.
Currently, some actuators are driven by piezo-electric elements which are connected to a frame at one end and to a movable member at the other end. In such actuators, the expansion and the contraction of the piezo-electric element moves a movable member in an expanding and contracting direction of the piezo-electric element. The movable member can thus be used to advance and retract members such as printing wires through transmissional facilities.
The piezo-electric element used in the above-mentioned actuators generally consists of laminated piezo-electric ceramics that linearly expand in the longitudinal direction of expansion and contraction.
However, negative longitudinal linear expansion of the piezo-electric element caused by changing circumjacent temperature effects the degree of the expansion and contraction of the piezo-electric element and consequently effects the degree of the motion of the movable member.
In order to compensate for the negative linear expansion of the piezo-electric element, a temperature compensation member 6 with a positive linear expansion is inserted between one end of a piezo-electric element 5 and a frame 1, as shown in FIG. 5.
The length L of the temperature compensation member 6 sufficient to compensate for the negative linear expansion of the piezo-electric element 5 is calculated by the following equation; EQU 11.7.times.10.sup.-6 .times.L-6.0.times.10.sup.-6 .times.18
=1.2.times.10.sup.-6 .times.(L+18)
where the piezo-electric element 5 is 2 mm long, 3 mm broad and 18 mm high and composed of laminated piezo-electric ceramics whose negative longitudinal linear thermal expansion coefficient is -6.0.times.10.sup.-6 /.degree.C., the frame 1 consists of invar alloy whose linear thermal expansion coefficient at composition 36Ni-Fe is 1.2.times.10.sup.-6 /.degree.C., and the temperature compensation member 6 consists of SUS alloy whose linear thermal expansion coefficient is 11.7.times.10.sup.-6 /.degree.C. The calculated length L of the temperature compensation member 6 according to the equation is approximately 12.34 mm. Thus, the actuator must be large enough to accommodate the length L of the temperature compensation member 6, and its weight is accordingly heavy. Moreover, the heat generated by the piezo-electric element 5 cannot be conducted to the temperature compensation element 6 quickly enough precisely to compensate for temperature variations.